Processing magnetic material



United States Patent Ofifice 3,024,1 3]. PRUCEEESING MAGNETIQ MATERIALRobert E. Burlret. Breckenridge, and William Clark Coy and Lloyd E.Peter-man, Leechbnrg, Pa, assignors to Allegheny Ludlum SteelCorporation, Eraclrenridge,

Pan, a corporation of Pennsylvania No Drawing. Filed Aug. 2, 1960, Ser.No. 46,904

3 Claims. (Cl. 140-100) This invention relates to a process forproducing ductile magnetic alloys and in particular to the heattreatment for producing substantially pure iron-cobalt magnetic alloysin a ductible condition.

In the past there have been a number of soft magnetic alloys which havebeen developed to provide an optimum combination of magnetic properties.Among these, an alloy known in the trade as 2V Permendur is noteworthyin that it is characterized by the highest operating magnetic induction,the highest Curie point and the highest magnetostriction of any of thecommercially available soft magnetic alloy core materials. Recently, avariation of Permendur has been announced consisting of a substantiallypure alloy comparing about 2% vanadium, 49% cobalt and 49% iron. Thissubstantially pure magnetic alloy is known to the trade as Supermendur.It is characterized by having a total impurity content of less than0.10% impurities and, when properly processed, has the lowest coerciveforce, the lowest hysteresis loss, the highest maximum permeability, thehighest permeability associated with the highest flux density, thehighest remanence, and a rectangular hysteresis loop with the greatestflux swing from minus remanence to plus saturation, as compared withother known and commercially available soft iron-cobalt magnetic alloycore material. However, these magnetic properties can only be obtainedwhen the alloy is made from proper raw materials and is subjected to theproper processing.

In the normal mill processing of the commercially available Permendur,the hot rolled material is annealed at a temperature of about 1670 F.and quenched in ice water to obtain a malleable structure suitable forcold rolling. Since the chemical composition of Supermendur varies fromthe chemical composition of Permendur essentially only in the impuritycontent present in the alloys, it was believed that the heat treatmentapplied to Permendur would be effective when applied to Supermendur forproducing a structure which would permit subsequent cold rolling to thefinal gauge. Experience has shown, however, that such is not the case;Supermendur, who so processed, became extremely brittle and cracked uponnormal withdrawal from the furnace into the quenching medium.

An object of this invention is to provide a process wherebysubstantially pure iron-cobalt soft magnetic alloys can be heat treatedto obtain a malleable structure.

Another object of this invention is to provide a process in whichsubstantially pure iron-cobalt soft magnetic alloys can be processed ona commercial scale.

A more specific object of this invention is to provide a process forcommercially treating substantially pure iron cobalt-vanadium softmagnetic alloys to obtain the highest maximum permeability associatedwith the low coercive force and hysteresis loss.

Other objects of this invention will become apparent from the followingdescription:

In a broad sense, the process of this: invention relates to thesubstantially pure iron-cobalt alloys to which small amounts of otherelements such as titanium, chromium or vanadium may be added, an exampleof which is the alloy known as Supermendur and which has a. compositionwithin the range between 1.5% and 2.5% vanadium, 45%

and 52% iron and 45 and 52% cobalt with not more than 0.10% incidentalimpurities exclusive of nickel which may be present in an amount up to0.4% as will be referred to hereinafter. In this type of alloy, it isneces- Sary that the highest purity be obtained in the final chemicalanalysis of the alloy. Presently this is accomplished by vacuum meltinghigh purity vanadium, electrolytic iron, and electrolytic cobalt.

It is to be noted that despite the use of extremely pure componentscertain other elements are normally present within the alloy,Supermendur. Thus even by employing electrolytic cobalt as the source ofcobalt in the present alloy, the resulting alloy may contain nickel inan amount.

of up to 0.4%. It is not economically feasible to remove the minuteamount of nickel from the cobalt by any known commercial means; however,in this particular alloy, this minute amount of nickel, that is, up to amaximum of 0.4% is not essential and may be present without adverselyaffecting the magnetic characteristics of the alloy to any seriousdegree. However, there are other elements which also appear in the finalanalysis of the alloy which are partially introduced through the rawmaterials and partially through contamination from the furnacerefractories. So long as these incidental impurities, as that term isused herein are limited to 0.10% maximum, exclusive of the nickelcontent, the alloy is capable of exhibitirrg outstanding magneticcharacteristics, which magnetic characteristics are vastly superior tothose of the Permendur, regardless of whether each of the alloys hasbeen subjected to a regular anneal or a special magnetic annealing heattreatment as will be set forth more clearly hereinafter. These highpurity components are melted in a controlled atmosphere furnace or in avacuum type furnace and are cast into ingots in the normal steel makingmanner. 7

After the metal has solidified and cooled to a sufficient degree, it isremoved from the controlled atmosphere or vacuum furnace and thereaftercooled in air to room temperature. The ingots are then heated to atemperature in the range between 2000 and 2250 F. and preferably forgedto disrupt and distribute the as-cast structure.

It will be appreciated however, that other forms of hot working may beemployed as desired. No particular difiiculty has been encountered inthe hot working of these alloys. Moreover, it has been found that if itis necessary to condition the. initially hot worked alloy, scarfing ispreferred since it has been found that grinding may have a tendency toproduce hairline cracks which may cause trouble during any subsequenthot working operation.

The initially hot worked alloy can be hot rolled in the regular steelmill manner. It is preferred to heat the alloy to a temperature of about2250 F. and complete the hot rolling operation before the material dropsto a temperature below 1700 F. In practice it has been found that afterforging the ingot into a slab measuring 2% x 4 /2 x 7', the slab may beconveniently hot rolled to a semifinished mill product termed a hotrolled band having a thickness between 0.060" and 0.090" by heating theslab to a temperature of about 2250 F. and hot rolling directlytherefrom to a hot rolled band, the finishing temperature being between1700 F. and 1750" F. The hot rolled band must be coiled hot since duringthe cooling of the hot rolled band to room temperature, the alloybecomes brittle to such a degree that it cannot be coiled cold.

Subsequent to the hot rolling treatment, it is necessary to heat treatthe hot rolled band in order to produce the band in a malleablecondition suitable for cold Working to the final gauge. In the prior artpractice, as discussed in the publication Ferromagnetism, by Bozorth,page 202, D. Van Nostrand Co., 1951, in processing Permendur the hotrolled band is heat .treated by annealing at a Patented Mar. 6, 1962.

temperature in the range between 1550" F. and 1650 F. and thereafterquenching the iced brine. On a commercial scale, the coil of hot rolledmaterial is placed on a reel inside of a furnace having first beenjoined as by welding or bolting to a stainless steel leader strip whichprojects out of the furnace. After annealing at 1550 F. to 1650 F. for atime period of between 4 and 16 hours, the material is withdrawn fromthe furnace and quenched by pulling it through a quenching tubcontaining iced brine. The heat treated band can be coiled and it is ina sufiiciently malleable condition to withstand a great degree of coldwork. Subsequent to the quenching operation, the annealed hot rolledband is surface conditioned, pickled and cold rolled to thickness ofbetween 0.001 and 0.014" without any intermediate heat treatment.

When the above process was applied to Supermendur, it was found that thematerial cracked and fractured when removed from the furnace and drawninto the quenching medium. Metallographic examination indicated that thematerial underwent excessive grain growth. It is therefore necessary tofind .-a means for controlling the grain growth of this material sinceit has been found that the material exhibiting an extremely large grainsize, that is, greater than about ASTM #3, is impossible to quench to amalleable structure on a commercial scale. In this same vein, Permendurwhen processed in accordance with the prior art practice as set forthhereinbefore did not exhibit the extremely large grain size.Metallographic examination reveals that his may be due in part to thefact that commercially melted Permendur exhibits a large amount ofinclusions which X-ray studies indicate are predominantly A1 and V 0 Thetotal weight percentage of these inclusions in commercially meltedPermendur was found to be greater than 0.10%. On the other hand, thesubstantially pure alloys like Supermendur to which this invention isapplicable and which are made from the substantially pure raw materialshave a total weight percentage of inclusions or impurities of less than0.10% contained therein exclusive of the nickel content.

In order to more clearly demonstrate the advantages of the process ofthis invention, reference may be had to Table I which illustrates thecomposition of a number of alloys some of which fall within the range ofthe analysis of Supermendur, and others which fall within the range ofPermendur.

TABLE I Chemical Composition [Weight percent] Element 2V009 2V0l1 2V0l230571 NOON! O into a single 9" x 9" ingot weighing 347 lbs. The ingotwas heated to 2250 F. and forged to a slab measuring 2%" x 4%" x 7. Theslab was heated to 2250 F. and hot rolled directly Without difiiculty toa band measuring 4%" x 0.056", the finishing temperature being 1730 F.Good surface and very little edge checking was noted. The material wascoiled hot and thereafter allowed to cool to room temperature.

Samples were cut from the hot rolled band of Heat 2V009 and suchsamples, together with samples of Permendur, the analysis of which isnot reported herein, were subjected to laboratory experiments todetermine the quenching temperature range that should be applied to thematerials. As quenched from. a temperature in the range of 1450 F. to1700 F., both materials could be bent without difliculty and exhibitedhardnesses in the range from 92 R to 23 R in the laboratory quenchedsamples. As with past practice, it was therefore decided that atemperature of 1600 F. should be used for the annealing of this material(Heat 2V009) since it reacted on the laboratory scale in the same manneras Permendur. The hot rolled band of the Heat 2V009 material with itsstainless steel leader strip was positioned within the annealing furnaceand heated to a temperature of 1600" F. and held for three hours. Itrequired six hours to heat the coil from 1115 F. to 1540 F. and the timeabove 1540 F. was about 17 hours. Simultaneously and in the samefurnace, a regular Permendur band was also heat treated and it wasWithdrawn first and successfully quenched into a 10% ice brine quenchingmedium held at a temperature between 30 F. and 35 F. However, when theSupermendur coil was pulled from the furnace, it started to crack andbroke and shattered after about 20 feet were out of the furnace. Thematerial from. the Supermendur coil was brittle and unusable forcomrnercial processing. It was interesting to note that the Supermendurbroke during and after the quench and the break did not necessarilystart at the edge. Metallographic examination also revealed that thematerial as hot rolled had essentially the same grain size as theregular Permendur. However, it was evident that the Supermendur was muchcleaner and contined far less inclusions than the regularly producedPermendur. During the mill anneal, the Permendur underwent substantiallyno grain growth, whereas the Supermendur developed a substantiallylarger grain size. It was therefore believed that the unsuccessfulquenching of the Supermendur was due primarily to the large grain size.

In order to alleviate this condition and to produce a malleablestructure which permits cold working to the finish gauge of the finalproduct, this invention utilizes a heat treatment including, inter alia,preheating the alloy to a temperature in the range between about 1020 F.and 1120 F. for a time period ranging between about 3 hours and 10hours, followed by another heat treatment,- without any intermediatecooling, at a temperature in the" range between 1425 F. and 1475 F. fora time period ranging between 1 /2 hours and 3 hours, followed by aquenching treatment to a temperature in the range between 35 F. and 50F. after which the temperature of the alloy is permitted to raise toroom temperature. Where desired, the alloy can be thereafter heated to atemperature in the range between room temperature and about the boilingpoint of water to aid in coiling the quenched alloy.

The necessity of preheating the essentially pure ironcobalt alloy at atemperature within the range between 1020" F. and 1120 F. will permitthe alloy to be later heated at the higher temperature without thedanger of excessive grain growth. This appears to be of paramountimportance in the successful quenching of Supermendur because the highpurity of the metal permits rapid grain growth at the heat treatmenttemperature. Conversely stated, since the alloy Supermendur has lessthan 0.10% of incidental impurities exclusive of nickel, the alloy is.

relatively free from inclusions, and since the inclusions are a greatcontributing factor for inhibiting grain growth at the heat treatment.temperature, the absence thereof contributes to the observed phenomenonof excessive grain growth at the heat treatment temperature. It must berealized that primarily grain growth occurs during the heat treatment ofthis alloy. The heat treatment of this essentially pure iron-cobaltmagnetic alloy requires heating to a temperature above the criticaltemperature of the orderdisorder phenomenon which occurs at atemperature in the range between about 1290 F. and 1380 F. dependingupon the composition. It has been noted that when the material is heatedabove the critical transformation temperature of the order-disorderphenomenon, excessive grain growth occurs which it is believed ismotivated by grain boundary energies. It is thus apparent that the timeat which the material is held at a temperature above the criticaltransformation temperature must be held to a minimum in order tominimize excessive grain growth, because the excessive grain growthappears to make it impossible to commercially quench the alloy. For thisreason, it is preferred to preheat the alloy to a temperature slightlybelow the critical transformation temperature and the temperature atwhich rapid grain growth occurs and preferably at a temperature in therange between 1000 F. and 1120 F. Since no appreciable grain growthtakes place at this temperature, it is preferred to hold the alloy inthis temperature range until it is thoroughly soaked and preferably fora time period ranging between about 3 hours and about hours. Thepreheating of the alloy will permit the alloy mass to be heated to thistemperature range without excessive grain growth occurring, therebyminimizing the total time at which the alloy is held at a temperatureabove the critical transformation temperature and the temperature atwhich rapid grain growth occurs. It is, of course, realized that it isnecessary to heat the alloy to a temperature above the criticaltransformation temperature in order to obtain an optimum amount ofdisorder which will permit the alloy to be successfully quenched toobtain a malleable structure.

Following the preheating of the alloy as set forth hereinbefore, thetemperature of the essentially pure ironcobalt alloy is raised to atemperature preferably within the range between about 1425 F. and about1475 F. for a time period of between about 1 hour and about 4 hours. Ofcourse, it will be appreciated that the alloy is not cooled subsequentto the preheating for such cooling would destroy the advantage gained bypreheating. It is during the heat treatment at this higher temperaturethat the alloy undergoes the order-disorder transformation and graingrowth. However, since the temperature of the heat treatment is near thelower limit of the range of temperature in which rapid growth occurs,and the time that the alloy is subjected to this temperature range isrelatively short, grain growth is minimized. While lower temperaturesmay be employed for longer time periods or higher temperatures may beused for shorter time periods, optimum results are obtained when thetemperature is maintained within the range between about 1425 F. andabout 1475 F. for a time period of about 1 hour to about 4 hours.Thereafter, the alloy is quenched as will be fully explainedhereinafter.

It is preferred to immediately quench the alloy from the heatingtemperature described in a quenching medium maintained at a temperaturein the range between about 30 F. and about 50 F. This is convenientlyaccomplished by subjecting the alloy to immersion in a refrigeratedwater solution containing about 10% brine. When the alloy reaches atemperature within the range between 30 F. and 50 F., it is removedtherefrom, and where desired, it is heated to a temperature in the rangebetween about 80 F. and about 190 F. to facilitate easier coiling.

In practice, Heat 2V011, having the analysis given hereinbefore, wasforged and hot rolled to a thickness of about 0.060 inch. The metal incoil form was placed in a furnace and preheated to a temperature in therange between 1000 F. and 1050 F. for a time period of about 10 hours.Thermocouple measurements indicated that the coil attained a temperaturein the range between 970 F. and 990 F. for a time period of about 7hours. The temperature was then rapidly increased to a temperature inthe range between 1425 F. and 1465 F. for a time period of 2 hours. Thecoil was then withdrawn through a quenching medium consisting of a 10%solution of brine held at a temperature of 50 F. The coil was withdrawnat a rate of speed in the range between and 248 feet per minute. Testsrevealed that at room temperature the heat treated metal could be bentwithout fracture and exhibited a hardness of between 94 R and R Heat2V012, having analysis given hereinbefore in Table I, was processedusing the same cycle as set forth for Heat 2V011. Substantially similarresults were obtained in that the metal could be bent 180 withoutfracture and exhibited a hardness in the range between 93 R and 97 R Theprocess of this invention is effective for commercially quenchingsubstantially pure iron-cobalt magnetic alloys Without producing anyadverse eflect upon the magnetic characteristics thereof. The alloyafter quenching, according to the process of this invention, is in asufficiently ductile condition, such that it can be cold rolled to athickness in the range between 0.001 and 0.014 inch in thickness. Themetal, having the thickness as above set forth, is then subjected to abright anneal in a magnetic field in order to develop thecharacteristics. It has been found that when the metal of Heats 2V009and 2V011 was bright annealed at 1600 F. for 4 hours in a DC. magneticfield of 6 oersteds and thereafter cooled at the rate of about 200 F.per hour, these steels had the following characteristics:

TABLE II Section A HEAT 2V009-4 MIL WOUND CORE 1H SH 1011 Em", gausses21, 050 22,100 22, 500 B1, gausses 20, 550 20, 700 740 He. oersteds..196 .196 .192 rlBmaxiuh .976 .937 922 Hma; 85, 653

Section B HEAT 2V01112 MIL WOUND CORE-MgO COATED Bmux, guesses 20, 55021, 800 22, 300 Br, gausses 550 19, 800 19, 800 He, oersteds.. .17 .177.177 T max .951 .908 89 pm 85, 000

As can be seen from Table II, the magnetic properties exhibited by thealloy are reproducible within narrow limits from heat to heat. Theseproperties are as good as the properties reported for the metal whenprocessed in a laboratory under exacting conditions and control.

Reference is directed to Table III, which illustrates the effect ofindicidental impurities on various magnetic characteristics of thesealloys.

7 TABLE III Permendur Versus Sapermendur; Standard Anneal VersusMagnetic Anneal Ring Samples [Annealed at 1385 FA hours PDH-z rapidcool] Pcr- Super Super- Pcr- Supermendur Inendur men (1111 mendur mendurHeat 30571 2V018 1V120 30571 2V018 Gauge 014 014 .014 006 .000 am from10 2,800 10,000 8, 000 4, 200 8, 200

A. D.C. HYSTERESIS FROBI 1H B'nz 1, 670 4, 250 2, 260 1, 900 4, 000 2702, 150 920 3 2, 200 164 403 420 192 500 162 506 407 189 550 B. D.C.HYSTERESIS FROIVI 5H D.C. HYSTERESIS FR OM H D.C. HYSTERESIS FROM 25H E.D.C. HYS TERESIS FROM 5011 Magnetic Anneal Permendur Supermondur A. D.C. HYSTERE SIS FROM 1111 B. D.C. HYSTERESIS FROIW 5H G. D.C.I-IYSTERESIS FRONI 10H 10, 700 22, 200 Em 12, 300 21, 000 1. 72 240Br/Bm .737 946 D. D.C. HYSTERESIS FROM 25H 19, 900 22 700 1 3: 10, 900211000 H 1.70 .238 Br/Bm 548 925 E. D.C. HYSTERESIS FROM 50H 21 400 22,900 B m 10 ,800 20, 900 l. 76 235 505 913 By comparison with thechemical analyses reported in Table I hereinbefore, it will be seen thatsubstantially the only difference between Heats 30571 and Heats 2V018and 1Vl20 resides in the chemical composition of the incidentalimpurities, exclusive of nickel. Heat 30571 is regular Permendur,whereas the other heats are Supermendur having a maximum of 0.10%incidental impurities. Table III illustrates that the Supermendur heatsexhibit outstanding magnetic characteristics in comparison with thePermendur characteristics where they have been given a standard anneal.Thus, the sole difference residing in the magnetic characteristics ofthese alloys is clearly attributed to the amount of incidentalimpurities contained therein. This comparison of the magneticcharacteristics is more outstanding where these alloys have beensubjected to a magnetic anneal consisting of heating to a temperature of1400" F. at the rate of about 250 F. per hour, holding at 1400 F. for atime period between 15 minutes and one half hour and then heating to1550 F. for four hours. Thereafter, the alloys are cooled to 1000 F. ina magnetic field of 5 amperes. Throughout the heat treatment cycle thealloys are subjected to a protective atmosphere of pure dry hydrogen.The test results in Table III clearly illustrate the outstandingmagnetic characteristics exhibited by Supermendur which result primarilyfrom the purity of the alloy.

There are no special skills or equipment utilized in practicing theprocess of this invention. The equipment is the same that is used in thecommercial production of other iron-cobalt magnetic alloys which arecurrently in use. The process is effective for commercially quenchingiron-cobalt magnetic alloys having an extremely high degree of puritywhich are characterized by being subjected to excessive grain growthwhen heated to a temperature above the critical transformationtemperature.

This is a continuation-in-part of application Serial No. 769,561, filedOctober 27, 1958, now abandoned.

We claim:

1. In the processing of substantially pure iron-cobalt alloys in theform of strip to obtain a ductile alloy, characterized by the alloycontaining as impurities less than 0.4% maximum nickel and less than0.10% of other incidental elements, the steps comprising, preheating thealloy in the form of coiled strip to a temperature in the range between1020 F. and 1120 F. for a time period ranging between about 3 hours andabout 10 hours, increasing the temperature of the heat treatment untilthe temperature of the alloy is within the range between about 1425 F.and 1475 F., holding the alloy strip at said temperature range for atime period of between 1 /2 hours and 3 hours, and withdrawing the stripfrom the heat treating furnace through a quenching medium maintained ata temperature in the range between about 30 F. and 50 -F. at a rate ofbetween about and about 300 feet per minute.

2. In the processing of substantially pure iron cobalt alloys in theform of strip to obtain a ductile alloy, the alloy being characterizedby containing as impurities less then 0.4% maximum nickel and less than0.10% of other incidental elements, the steps comprising, preheating thealloy to a temperature in the range between 1020 F. and 1120" F. for atime period ranging between about 3 hours and about 10 hours, increasingthe temperature of the alloy to temperature range between about 1425 F.and 1475 F. for a time period ranging between about 1 /2 hours and about3 hours, quenching the alloy in a quenching medium maintained at atemperature in the range between about 30 F. and 50 F. and thereafterheating the alloy to a temperature of up to F.

3. In the processing of substantially pure iron-cobalt magnetic alloysto obtain a ductile alloy having a composition within the range between45% and 52% iron, 45% and 52% cobalt, 1.5% and 2.5% vanadium and whichis characterized by having as impurities less than 0.4% maximum nickeland less than 0.10% of other incidental elements, the steps comprising,preheating the alloy to a temperature within the range between 1020 F.and 1120 F. for a time period ranging between about 3 hours and about 10hours, increasing the temperature of the alloy to a temperature in therange between 1425 F. and 1475 F., holding the alloy at said temperaturefor a time period ranging between 1% hours and 3 hours, and quenchingthe heated alloy in a quenching medium maintained at a temperature inthe range between about 30 P. and 50 Fv References Cited in the file ofthis patent Bozorth: Ferromagnetism, 1951, D. Van Nostrand Co., Inc.,page 202.

' Metals Handbook, by the American Society for Metals, 1948, page 665.

Nesbitt: VicalloyA Workable Alloy for Permanent Magnets. Transactions ofthe American Institute of Metals Division. Volume 166, 1946, pages415-425,

10 page 422 particularly relied on.

1. IN THE PROCESSING OF SUBSTANTIALLY PURE IRON-CABALT ALLOYS IN THEFORM OF STRIP TO OBTAIN A DUCTILE ALLOY, CHARACTERIZED BY THE ALLOYCONTAINING AS IMPURITIES LESS THAN 0.4% MAXIMUM NICKEL AND LESS THAN0.10% OF OTHER INCIDENTAL ELEMENTS THE STEPS COMPRISING, PREHEATING THEALLOY IN THE FORM OF COILED STRIP TO A TEMPERATURE IN THE RANGE BETWEEN1020*F. AND 1120*F. FOR A TIME PERIOD RANGING BETWEEN ABOUT 3 HOURS ANDABOUT 10 HOURS, IN CREASING THE TEMPERATURE OF THE HEAT TREATMENT UNTILTHE TEMPERATURE OF THE ALLOY IS WITHIN THE RANGE BETWEEN ABOUT 1425*F.AND 1475*F., HOLDING THE ALLOY STRIP AT SAID TEMPERATURE RANGE FOR ATIME PERIOD OF BETWEEN 1/2 HOURS AND 3 HOURS, AND WITHDRAWING THE STRIPFROM THE HEAT TREATING FURNACE THROUGH A QUENCHING MEDIUM MAINTAINED ATA TEMPERATURE IN THE RANGE BETWEEN ABOUT 30*F. AND 50*F. AT A RATE OFBETWEEN ABOUT 160 AND ABOUT 300 FEET PER MINUTE.